The present invention relates to inlet control valves for use in pneumatic systems and, more particularly, to inlet control valves useful with pneumatic tools.
In a rotating pneumatic tool such as a drill motor, the electric motor found in electric tools is replaced by an air turbine motor. Compressed air is fed to the tool and passes through the turbine motor to drive the tool and, thereafter, is exhausted to the atmosphere. To provide ease of operation and prevent the continuous operation of the tool, a valve operated by a trigger mechanism is normally included within the inlet passageway of the tool conducting the pressurized fluid, usually air.
Until recently, such valves have been, in one example, of a single tilting plate variety which afforded simple on/off control. With the advent of solid state electrical controls, the advantages of variable speed in an electric tool rapidly became very obvious. It was found to be highly desirable to allow a drill, for example, to first rotate slowly to establish its point of entry and thereafter increase the speed at which the drill is operating. With electric controls, true variable speed controllable throughout the range between virtually no rotation and full speed rotation was possible by the use of a miniaturized solid state rheostat mechanism contained within the tool and operated by the trigger mechanism.
A similar problem exists with pneumatic riveting hammers such as those used to rivet the skin on aircraft. The riveter and his assistant are on opposite sides of the skin section. The assistant places a bucking bar against the inner end of the rivet and the riveter applies the rivet hammer to the rivet head. When the hammer is activated, if everything is properly aligned, a piston reciprocates and hammers the rivet against the bucking bar to its set condition. Unfortunately, the riveter and the assistant are on opposite sides of the skin and cannot see each other. It is not uncommon for the rivet hammer to be on one rivet and the bucking bar to be on another. Sudden application of full power can result in slipping of the tools and damage to the valuable aircraft skin. A variable speed operation would allow the team members to assure proper alignment of the tools on the same rivet before full setting power was applied.
Unfortunately, no such full-range variable speed control has been available for pneumatic tools. Recently, Eckman disclosed in his U.S. Pat. No. 4,258,799 a two-speed inlet valve implemented by merely doubling up the tilt valve used previously. The operation of Eckman's valve is shown in FIGS. 1-3. The valve, generally indicated as 10, comprises a first annular disc 12 having a second annular disc 14 covering a hole in the open center of the first disc 12. An operating shaft 16 extends downstream from the second disc 14 through the center of the first disc 12. A spring 18 biases the two discs 12, 14 against shoulder 20 of the passageway 22. Pressurized fluid is contained within passageway 22 attempting to flow in the direction of arrow 24. FIG. 1 shows the valve 10 in its "off" condition.
In FIG. 2, the valve 10 is shown in its first level (i.e. slow speed) "on" position. The trigger 26 has been depressed sufficiently to cause shaft 28 to contact the operating shaft 16 and tilt the second annular disc 14 away from the first annular disc 12 exposing the hole therein. In FIG. 3, the valve 10 has been opened to its second level (i.e. fast speed) "on" position. The shaft 28 has continued to force operating shaft 16 away from its normal longitudinal alignment into contact with the upper shoulder 30 surrounding the open center of the first annular disc 12 to, thereby, tilt the first annular disc 12 away from shoulder 20 thus fully opening the passageway 22. As can be seen, there is no variable speed operation, valve 10 is either "off", "on" at low speed, or "on" at high speed.
Wherefore, it is the object of the present invention to provide an inlet control valve particularly suited for such air operated tools providing true variable speed operation throughout the full speed range in the manner of electrically operated tools.